1. Field of the Invention
The present invention relates to a piezoelectric pump including a diaphragm that is deflected and deformed by a piezoelectric vibrator.
2. Description of the Related Art
In general, piezoelectric pumps including a diaphragm that is deflected and deformed by a piezoelectric vibrator are compact and have a low profile. In addition, such piezoelectric pumps have low power consumption. Accordingly, such piezoelectric pumps can be used as, for example, fuel transportation pumps of fuel cells. However, such piezoelectric pumps are required to have an increased discharge pressure and higher rates of flow of liquid, such as fuel, to be transported and an ability to discharge the air that enters a pump chamber to the outside.
A piezoelectric pump having an increased ability to discharge air (gas) that has entered a pump chamber to the outside is described in Japanese Unexamined Patent Application Publication No. 03-031589 and in Japanese Unexamined Patent Application Publication No. 2008-163902.
The piezoelectric pump described in Japanese Unexamined Patent Application Publication No. 03-031589 includes a casing having an inner surface in which a very small gap is provided between the casing and a piezoelectric vibrator when the amplitude of the piezoelectric vibrator is maximized during a pump compression time (an air ejection time). That is, the inner surface of the casing is configured so that the shape of the inner surface is substantially the same as the shape of the deflected piezoelectric vibrator when the amplitude of the piezoelectric vibrator is maximized.
The piezoelectric pump of Japanese Unexamined Patent Application Publication No. 2008-163902 is described next with reference to FIG. 1. FIG. 1 is a plan view of a piezoelectric pump P described in Japanese Unexamined Patent Application Publication No. 2008-163902. The piezoelectric pump P includes a pump body, an elastic film, a piezoelectric device 21, and a pressure plate 30. The pump body includes a concave portion 11 which is a portion of an inlet valve chest, a concave portion defining a pump chamber 12, and a concave portion 13 that defines an outlet valve chest. A connection passage (an inlet) 14 extends between the inlet concave portion 11 and the pump chamber 12. In addition, a connection passage (an outlet) 15 extends between the outlet concave portion 13 and the pump chamber 12.
The pressure plate 30 includes an opening hole 31 provided therein at a position corresponding to the piezoelectric device 21. An inlet port 34 includes an inlet check valve 40 that opens and closes the inlet port 34. In addition, an outlet port 35 includes an outlet check valve 41 that opens and closes the outlet port 35.
A base portion 16 is provided in the inner bottom surface of the pump chamber 12 so as to face the middle portion of the piezoelectric device 21. A flow passage portion 17 that communicates with the connection passage 14 and the outlet 15 is provided in the outer periphery of the base portion 16. Since a gap between the middle portion of the piezoelectric device 21 and the base portion 16 becomes narrow if the piezoelectric device 21 is deflected and deformed, liquid present on the base portion 16 is ejected out to the flow passage portion 17 on the periphery side. Thus, the air is trapped by the flow passage portion 17. In addition, as the volume of the pump chamber 12 is changed, the liquid in the flow passage portion 17 is ejected towards the outlet 15 and, therefore, the air is ejected together with the liquid.
In order to produce a low-profile piezoelectric pump, the diaphragm and the pump body are made of a thin elastic sheet. However, when the sheet is thin, it is difficult to process the sheet into a particular shape as described in Japanese Unexamined Patent Application Publication No. 03-031589. Accordingly, if an air bubble enters the pump chamber, the pressure generated by the pump decreases. Thus, the air bubble cannot be ejected out and, therefore, the operation of the pump may be stopped.
In addition, as described in Japanese Unexamined Patent Application Publication No. 2008-163902, in the configuration in which the flow passage portion arranged to trap air is provided on the inner periphery of the pump chamber, if the entire pump chamber is filled with air, the air can be effectively ejected (at a time of dry start). However, the piezoelectric pump is not always used such that the piezoelectric pump starts and continuously transports the liquid. The piezoelectric pump needs to have the ability to reliably eject gas and transport the liquid even when the piezoelectric pump is intermittently operated (e.g., the piezoelectric pump starts transporting the liquid and temporarily stops, and subsequently, the piezoelectric pump resumes its operation). However, if a piezoelectric pump having a structure described in Japanese Unexamined Patent Application Publication No. 2008-163902 operates intermittently, it is difficult for the piezoelectric pump to provide sufficient pressure.